An organic electroluminescent (EL) display, being a kind of light-emitting apparatus, is a new type of flat panel display and formed from an array of organic light emitting diode (OLED) devices. In general, an OLED device is comprised of an electrode serving as the anode, an electrode serving as the cathode and several thin organic layers sandwiched between these two electrodes. The organic layers include at least one emission layer. The emission layer is formed of a fluorescent organic compound, phosphorescent organic compound or a light emitting materials such as a quantum dot (QD) to emit light at each emission color. Applying voltage to the OLED device, holes and electrons are injected from the anode and the cathode, respectively, and form excitons in the emission layer. Then these excitons recombine and release their energy as emission of light.
One of the tasks to be accomplished in development of such an organic light-emitting apparatus is improvement of the emission efficiency. The OLED device generally has such a structure that an anode, an organic layer including a emission layer, and a cathode are one-dimensionally stacked. At this time, the refractive index (approximately 1.7 to 1.9) of the emission layer is larger than the refractive index of air. Therefore, most of light emitted from the inside of the emission layer is totally reflected at an interface of the stack film at which a high refractive index changes to a low refractive index. The totally reflected light becomes guided-wave light propagating in a horizontal direction of a substrate, and then is confined inside the OLED device. The ratio of light which can be extracted for use to the outside (light extraction efficiency) is generally only approximately 20%.
Therefore, in order to improve the emission efficiency of the organic light-emitting apparatus, it is important to improve the light extraction efficiency. Among conventional technologies, for example, “Appl. Phys. Lett., 69, 1997 (1996)”, “Appl. Phys. Lett., 81, 3921 (2002)”, and “Appl. Phys. Lett., 88, 073517 (2006)” each describe that, when a cavity structure is introduced into the OLED device to make use of an interference effect, the light extraction efficiency can be improved.
In addition to the conventional technologies described above, for example, Japanese Patent No. 2,991,183 (Japanese Patent Application Laid-Open No. H11-283751) proposes a method involving providing a periodic structure (such as photonic crystal or diffraction grating) in an upper or lower portion of an organic layer (on a light extraction side or on a side opposite thereto), in order to prevent total reflection to thereby suppress light confinement inside the OLED device.
The conventional technologies described in each of “Appl. Phys. Lett., 69, 1997 (1996)” and “Appl. Phys. Lett., 81, 3921 (2002)” have a problem that when the interference effect of the cavity is enhanced to improve the light extraction efficiency, a viewing angle dependency of a light emission pattern of the OLED device becomes larger, resulting in a change of emission color depending on the viewing angle.
Even in the conventional technology described in Japanese Patent No. 2,991,183 (Japanese Patent Application Laid-Open No. H11-283751), when the periodic structure is provided to improve the light extraction efficiency, there is a problem that the viewing angle dependency of the light emission pattern of the OLED device becomes larger due to wavelength dependency of a diffraction effect, resulting in a change in emission color depending on a viewing angle.